The present invention generally relates to processing received communication signals and particularly relates to reducing interference present in the received communication signals.
In many CDMA (Code Division Multiple Access) systems, the downlink spread spectrum signal is composed of multiple user signals that were spread with spreading codes designed to be orthogonal. As a result, when the channel is flat, the combined transmit and receive filters have a Nyquist pulse shape, and an ideal sample time is used, interfering components are not seen at the output of the demodulator. In general, the ideal sample time used to produce the samples coincides with the point in the Nyquist pulse that does not incur interference from adjacent pulses. In the case of a CDMA system, the adjacent pulses are the chips which make up the received symbol. If the actual sample time deviates from the ideal sample time, then the orthogonality of the multi-user signals is lost in the processing, and multi-user interference appears at the output of the demodulator.
Because the receiver does not know where the ideal sample times are relative to the received signal, conventional receivers typically over-sample the received signal and select the sample time corresponding to the best sample as the “ideal” sample time. For example, a wideband wireless receiver in a WCDMA (Wideband Code Division Multiple Access) system may over-sample the received signal at a sample rate equal to four times the chip rate, (four samples per chip). In order to produce the chip-spaced (or chip rate) samples for despreading, the wireless receiver selects the chip-spaced samples in the oversampled sample stream which are sampled closest to the ideal sample time. E.g., suppose succeeding samples in a four times oversampled sample stream modulo 4 are numbered as 0, 1, 2, 3, 0, 1, 2, 3, 0, 1, 2, 3, etc. Further, suppose that the sample time of the number 2 samples is closest to the ideal sample time of the chip pulses. In this scenario, the sample time corresponding to the number 2 samples is chosen as the best sampling time.
However, because of the limited number of samples, even the best sampling time may be offset from the ideal sample time by some amount, referred to herein as sample time error. In the over-sampled WCDMA example above, the sample time error may be as much as Ts/2, where Ts represents the sample period. Because the samples are acquired using non-ideal sample times, the orthogonality between users has been compromised, causing the samples to include interference from adjacent pulses. As a result, the sample time error typically degrades the overall performance of the receiver.